why boiler steam drum top is connected to Level Transmitter HP side & bottom is connected to Level Transmitter LP side ?
We use wet leg level measurement, where both tubes running from the drum to the level transmitter are filled with liquid water.
Here high pressure port (HP) on the DP (differential pressure) transmitter should be connected to the top of the drum and the low pressure port connected to the bottom.
Because the measurement is from the top down - if the vessel fills with cold water up to the top tap (HP), the ports of the DP cell will have the same water head above them so the DP will be zero.
Falling level will raise the measured DP - the pressure on the low tap (LP) will fall relative to that of the high tap still full of liquid.
Reference : crossco